#SAQs SOME SHORT ANSWER QUESTIONS ON Mixed venous blood and O2 tension and CO2 QUESTIONS A. List the normal values for mixed venous blood gases and briefly explain the factors determining mixed venous oxygen tension? B. Briefly describe the factors that influence the partial pressure of oxygen in mixed venous blood? C. Why does PvO2 not lie on the Standard Oxygen Dissociation Curve? D. What factors influence the mixed venous CO2 tension. Briefly explain how these factors exert their influence? E. What is Ficks Principle? F. What happens when Oxygen Dissociation Curve is Shifted To The Right? ANSWERS A. MIXED VENOUS BLOOD Mixed venous blood - mixture of all the systemic venous blood draining from all the tissue capillary beds of the body, excluding shunted blood (i.e. central or peripheral shunt). Pulmonary venous blood is not included. It Has 3 major component: 1. superior vena cava 2. inferior vena cava 3. coronary sinus Mixed venous blood is best taken from pulmonary artery as adequate mixing has occurred. Sometimes right ventricle can be used. In right atrium, the bloods are not adequately mixed. B. FACTORS AFFECTING PO2 OF MIXED VENOUS BLOOD 1. SOURCE • Coronary Sinus The heart has a High O2 extraction ratio (55-65%) The PO2 of coronary sinus blood is thus Typically low: -> 20mmHg Increase in myocardial oxygen consumption can only be met by increasing coronary blood flow • PO2 of SVC is higher than IVC PO2 of SVC and IVC PO2 from IVC is normally higher (SO2 77%) than from SVC (SO2 71%) => because kidney takes 25% of cardiac output but use only 7-8% of bodys O2 consumption => IVC receives blood more oxygen rich. Important With severe haemorrhage, => PO2 from SVC may be higher because of renal vasoconstriction. C. Why does PvO2 not lie on the Standard Oxygen Dissociation Curve? The PvO2 and SvO2 of mixed venous blood has a typical value of 40mmHg or 75% oxygen saturation This mixed venous blood point does NOT lie on the standard oxygen dissociation curve because at mixed venous blood level, the curve is right-shifted because of • increased PvCO2 • decreased pH CvO2 = 15mL/100mL D. PvCO2 and SvCO2 of mixed venous blood Typical value: 46mmHg or 52mLs of CO2 per 100mL blood => (assuming SvO2 of 75%) If SvO2 is 97%, => at PvCO2 of 46mmHg, CvCO2 = 50mLs/100mL => due to Haldane effect NB PaCO2 = 40mmHg CaCO2 = 48mLs/100mL E. What is Ficks principle? Answer It is the Amount of O2 extracted from respired gases equals the amount added to the blood that flows through the lung i.e. O2 consumption per unit time = O2 taken up by pulmonary blood flow per unit time Fick equation => VO2 = Q (CaO2 - CvO2) VO2 = O2 consumption per minute (mL O2/time) Q = pulmonary blood flow (mL/time) CaO2 = O2 concentration in blood leaving lung (mL/100mL) CvO2 = O2 concentration in mixed venous blood (mL/100mL) Factors affecting mixed venous O2 tension From Fick equation VO2 = Q x (CaO2 - CvO2) --> CvO2 = CaO2 - VO2/Q --> SvO2 = SaO2 - VO2/(Q x 1.34 x [Hb]) NB: SvO2 is derived so O2 dissociation curve (which is SpO2 vs PO2) can be used F. What happens when Oxygen Dissociation Curve is Shifted To The Right? Answer Firstly When O2 dissociation curve is fixed: SvO2 = SaO2 - VO2/(Qx1.34x[Hb]) SvO2 is increased when: • SaO2 is increased • O2 consumption (VO2) is decreased • cardiac output (Q) is increased • Hb concentration is increased As SvO2 increase, PO2 is increased. However, at mixed venous blood level of PO2 (40mmHg), changes in SvO2 doesnt have as great an effect on PvO2 as it would at higher level of PO2. So When O2 dissociation curve shifts to the the right given the same SvO2 --> PvO2 will increase PvO2 increase when ODC moves to the right due to: • increased PvCO2 • increased [H+] (i.e. drop in pH) • increased temperature • increased red cell 2,3 DPG G. What is the Bohr effect? Answer The Bohr effect denotes CO2 loading assisting in O2 unloading from Hb (for comparison, Haldane effect is when O2 unloading from Hb helps with CO2 loading) Majority of Bohr effect is due to pH change caused by changes in PO2) H. What are the Factors affecting mixed CO2 tension? According to Ficks principle Production of CO2 = Elimination of CO2 VCO2 = Q (CaCO2-CvCO2) => CvCO2 = CaCO2 - VCO2/Q So 1. When CO2 dissociation curve is fixed CvCO2 is increased when CaCO2 is increased --> Heavily influenced by alveolar ventilation CO2 output is decreased cardiac output is increased NB. CO2 output and production are the same in steady state, but different in dynamic state because much of CO2 produced is diverted into body stores. 2. When CO2 dissociation curve shifts to the right When CO2 dissociation curve moves, given the same SvCO2, PvCO2 will increase --> PvCO2 increase when dissociation curve moves to right due to: ? increased PvO2 ? decreased [Hb] SUMMARY Mixed venous O2 tension is increased by: increased SaO2 decreased O2 consumption increased cardiac output increased Hb concentration right shift in ODC, due to: * increased PvCO2 * increased [H+] * increased temperature * increased red cell 2,3DPG
Posted on: Thu, 06 Nov 2014 20:49:48 +0000